This invention relates to the generation of clock signals where it is desirable to spread the spectrum of the clock signal to minimize radio frequency interference (RFI), or more generally referred to as electromagnetic interference (EMI).
Virtually all digital circuitry employs clocks. A clock is a repetitive waveform that synchronizes the flow of data through a circuit or parts of a circuit. Being a repetitive waveform and often driving many blocks of circuitry, the frequency of the clock tends to radiate from the circuit and cause interference to outside devices and to reception of RF signals, for example. Allowed limits for spurious radiation of this type are set by government and industry standards. RFI can also interfere with nearby circuitry in the same system, such as radio receivers.
A known technique called “spread spectrum clocking” ameliorates but does not eliminate this problem. Spread spectrum clocking modulates the clock frequency and thereby spreads the spurious power across a range of spectrum, reducing the effect at any one frequency. The total energy cannot be reduced by modulation because the amount of energy is fixed; it can only be spread.
Typically the clock frequency is modulated by a 50 KHz triangle wave over a range of −0.6% to +0.0% of the unmodulated frequency. The modulating frequency must be above the audio range to avoid interfering with audio systems through various modalities. Other than that, the spread spectrum modulating frequency is kept as low as possible because of the difficulty of generating the clock waveform with the conventional Phase-Locked-Loop (PLL) methods, and because the clock is often upconverted to a higher frequency clock by a second PLL that cannot follow rapid modulation.
Spread spectrum modulation is usually not allowed to generate instantaneous frequencies above the nominal frequency, hence the +0.0% limit. Generally, the nominal frequency is the highest frequency at which the clocked circuitry will operate reliably and cannot be exceeded. However, some small-amplitude spread spectrum modulation is symmetric about the nominal frequency, taking advantage of allowed error limits. The lower limit, e.g. −0.6%, is set as a compromise between spectral spreading and circuit performance. A −0.6% limit, for instance, results in a 0.3% loss of circuit speed.
A simple PLL cannot generate a spread spectrum clock. Instead analog techniques are used to “pull” the PLL back and forth around its locked condition. Numerous techniques are known in the art for various modulation waveforms and how to balance between spectral considerations and the practical considerations of PLL design. Waveforms close to triangular, but not exactly triangular, have been shown to be better than simple triangle waveforms. Nonetheless, simple triangle waveforms are most commonly used.